Effects of Substantial Spark Retard on SI Engine Combustion and Hydrocarbon Emissions

Experiments were conducted to determine the effects of substantial spark retard on combustion, hydrocarbon (HC) emissions, and exhaust temperature, under cold engine conditions. A single-cylinder research engine was operated at 20° C fluid temperatures for various spark timings and relative air/fuel ratios. Combustion stability was observed to decrease as the phasing of the 50% mass fraction burned (MFB) occurred later in the expansion stroke. A thermodynamic burn rate analysis indicated combustion was complete at exhaust valve opening with -20° before top dead center (BTDC) spark timings. Chemical and thermal energy of the exhaust gas was tracked from cylinder-exit to the exhaust runner. Time-resolved HC concentrations measured in the port and runner were mass weighted to obtain an exhaust HC mass flow rate. Results were compared to time averaged well downstream HC levels. Quenching experiments, with carbon dioxide injected at the exhaust valve seats, were conducted to quantify cylinder-exit HC levels. Engine operation with a relative air/fuel ratio 10% lean of stoichiometric resulted in the lowest observed HC emissions. Port HC oxidation ranged from 15% to 37% with additional HC reductions (40-50%) in the runner noted with after top center spark timings. Fuel-rich engine operation with secondary air injection into the exhaust port yielded the lowest HC levels and highest exhaust gas enthalpy observed.